Help with QNH

hi,
Was wondering if someone could help me with calculating the derated
take-off thrust, if there is a change to the QNH for a given actual
takeoff weight.
I know that the thrust setting depends on temp so if you have a Actual
TOW less than the RTOW it is possible replace the OAT reading to a
higher value and therefore allowing for a reduced thrust setting on
takeoff.
What I don't understand is how this is calculated. Also what happens if
there is a change in QNH. how is this taken into account in the
calculation.

sorry if its a bit vague but got to start somewhere.